The present invention relates to a direct-current motor and a method for manufacturing the direct-current motor.
Conventionally, direct-current motors have been proposed that include a stator, which has magnetic poles arranged along the circumferential direction, and an armature, which faces the stator in the radial direction. The armature includes an armature core, which has teeth (salient poles) extending in a radial pattern. Armature coils are wound around the armature core in such a manner that the armature coils pass through slots formed between the circumferentially adjacent teeth.
For example, Japanese Laid-Open Patent Publication No. 2007-116813 discloses a direct-current motor that includes an armature core around which armature coils are wound by distributed winding, in which the armature coils each extend over a number of teeth. Japanese Laid-Open Patent Publication No. 2004-88915 discloses a direct-current motor that includes an armature core around which armature coils are wound by concentrated winding, in which the armature coils are respectively wound around the teeth in a concentrated manner. In general, the direct-current motor including the armature coils wound by distributed winding is advantageous in reducing noise and vibration since excitation force generated by magnetism between the armature core and the magnetic poles is smaller as compared to the direct-current motor including the armature coils wound by concentrated winding. The direct-current motor including the armature coils wound by concentrated winding is advantageous in increasing the power output since the space factor of the armature coils is high.
However, in the direct-current motor including the armature core around which the armature coils are wound by distributed winding, coil end portions of the armature coils that project further axially outward than the axial end faces of the armature core overlap each other in the axial direction. Thus, the size of the direct-current motor is undesirably increased. When reducing the axial length of the coil end portions to prevent the size of the direct-current motor from increasing, the number of turns of the armature coils needs to be reduced, which reduces the power output of the direct-current motor.
In the direct-current motor including the armature core around which the armature coils are wound by concentrated winding, fluctuation of magnetic flux is rapid since the difference between the number of the magnetic poles and the number of the teeth (salient poles) is small. Thus, excitation force acts on the armature core during activation of the direct-current motor, thereby generating great vibration on the direct-current motor.